I'm fairly certain that some of these are actually really common, and I just don't read enough. Also, most of them are Greek or some other kind of mythological creature. Do mythological figures count as fantastic?
Leviathan
Sphinx
Harpy
Manticore
Phoenix is too common, isn't it.
Wyverns? Unless they're also super common and/or are the same thing as dragons?
Sirens
Gremlins???
Lemmasu (I think. I probably spelled it wrong too.)
In case I hallucinated ever hearing about lemmasu, jackalope?
An extra, just in case: brownies.
Ooh! Gnomes?
Satyrs. But I think they're fairly common.
Centaurs. Same.
Honestly, I probably just don't read enough. :/